1. Field of the Invention
This invention relates to a travelling wave-type optical modulator.
2. Related Art Statement
The inventors disclosed in Japanese Laid-open publication Kokai Hei 10-133159(JP A 10-133159) that the portion of a substrate, constituting a travelling wave-type optical modulator, below an optical waveguide is thinned up to e.g. 10 xcexcm or below, for the high speed operation of the optical modulator and the reduction of the product (Vxcfx80xc2x7L) of operating voltage Vxcfx80 by electrode length L in the optical modulator.
The inventors have intensely investigated the above-type optical modulators and found out that the travelling wave-type optical modulators tend to be degraded through the increase of the optical insertion loss.
It is an object of the present invention to, in a travelling wave-type optical modulator including such a partionally thinned substrate made of a electro-optic single crystal, decrease the product (Vxcfx80xc2x7L) of operating voltage Vxcfx80 by electrode length L and the optical insertion loss with keeping the velocity matching for a microwave signal and the travelling optical wave.
This invention relates to a travelling wave-type optical modulator comprising;
a substrate made of an electro-optic material,
an optical waveguide fabricated on the top surface of the substrate, and
electrodes for modulating an optical wave through the optical waveguide,
wherein the substrate is partially thinned from the bottom surface of the substrate to form a first thinned portion and a second thinned portion so that the thickness of the first thinned portion is set to be larger than the thickness of the second thinned portion, and the optical waveguide is positioned in the first thinned portion.
The phrase xe2x80x9cthe optical waveguide is provided in the first thinned portionxe2x80x9d means that at least the optical refractive index center of the optical waveguide exists within the first thinned portion.
The inventors have intensely studied the cause of the increase of the optical insertion loss when the substrate is partially thinned below the optical waveguide for the velocity matching of a microwave signal and a travelling optical wave, and found out the following facts. As mentioned above, for realizing the velocity matching of a microwave signal of 10 GHz or over and a travelling optical wave, the substrate is required to be partially thinned up to 10 xcexcm below the optical waveguide. The thus obtained thinned portion of the substrate is surrounded by environmental atmosphere having a much lower refractive index than that of the electro-optic single crystal constituting the substrate. As a result, the optical waveguide provided in the thinned portion is likely to be plain-formed. On the other hand, the mode field pattern of an optical fiber connected to the travelling wave-type optical modulator before its introduction to the optical modulator is perfect circle. Therefore, it may be that the discrepancy in the mode field pattern of the travelling optical wave in between the optical modulator and the optical fiber is increased, so the optical energy of the travelling optical wave is not efficiently traveled and thus, the optical insertion loss is increased.
If the thinned portion of the substrate is upsized for mitigating the inefficient optical energy travel, the velocity matching of the microwave signal and the travelling optical wave can not be almost realized and the product (Vxcfx80xc2x7L) of the operating voltage Vxcfx80 by the electrode length L is increased.
From this point of view, the inventors have conceived that two types of thinned portion, a first thinned portion and a second thinned portion, are formed adjacently in the substrate constituting the optical modulator so that the first thinned portion is larger than the second thinned portion in their thickness, and the optical waveguide is fabricated in the first thinned portion. In this case, the optical insertion loss of the optical modulator can be reduced with keeping the velocity matching of the microwave signal and the travelling optical wave and keeping the product (Vxcfx80xc2x7L) low.
Next, this invention will be described in detail as follows, with reference to the attaching figures.